Fix stylelint rules conflicting with the styled-components processor

[SBoudrias]

When using stylelint-processor-styled-components, stylelint default config complain about a few placeholder the processor use. I felt it'd be quite convenient to have those handled out of the box by the base configuration.

[MaffooBristol]

I was just about to ask about this. 👍for this being reviewed. Example line:

 55:19  ✖  Expected "dummyValue" to be "dummyvalue"        value-keyword-case

with

function isShowingStyle() {}
export default styled.div`
  // ...
  ${isShowingStyle}
`;

[underyx]

This one's missing I think:

"value-keyword-case": [
  "lower",
  {
    "ignoreTypes": ["$dummyValue"]
  }
]